This invention relates to a process to be executed with the microprocessor of a vehicular engine- or powertrain-control module for determining the volatility of a gasoline fuel for use in adjusting air-to-fuel ratio during cold engine start.
Despite the development of catalytic converters for the treatment of exhaust gases from automotive internal combustion engines and sophisticated computer-based controls for engine spark timing and air-to-fuel ratio management, there remains a need for improvement in reducing emissions during the starting and warm-up of a cold engine. A significant amount of carbon monoxide and unburned hydrocarbon emissions is exhausted to the atmosphere during the first minute or so of start up and operation of a cold gasoline-powered engine when the catalyst has not reached its light-off temperature. The actual temperature of a xe2x80x9ccoldxe2x80x9d engine may vary greatly depending upon the ambient conditions, but even a tropic environment does not heat an engine and exhaust system to their effective operating regime. During a cold start, the noble metal-based catalytic converter is not hot enough to affect the oxidation of incompletely burned fuel. Furthermore, it is difficult to adjust the air/fuel ratio in the engine because the oxygen sensor in the exhaust system is not warmed up, and there is no information available to the engine controller concerning the ratio of air to fuel being admitted into the cylinders for combustion.
The amount of liquid fuel being introduced into the engine can be controlled by the activation times of the engine""s calibrated fuel injectors. But during cold start, only a portion of the injected fuel actually vaporizes for efficient combustion, and that portion depends largely on the volatility of the fuel being delivered to the cylinders of a cold engine.
Commercial fuel volatility varies by season and location. Usually, lower volatility fuels are supplied in the summer and warmer locations. To avoid driveability problems at lower temperatures, higher volatility fuels are provided in winter. Such fuel volatility variation, however, is a cause of concern for the auto industry in regard to both emissions compliance and good driveability for customer satisfaction. Designers of automotive vehicles have to provide engine controls and calibrations that enable rapid and smooth engine start-up to satisfy the customer, and such controls and calibrations must also lead to ever lower exhaust discharges of carbon monoxide and incompletely burned fuel species. Indeed, the design of engine controls would have been simpler had the actual volatility of the fuel used in the vehicle were known to engine controller in real time.
A well-recognized measure of fuel volatility is the familiar Driveability Index (DI)
DI=1.5T10+3T50+T90
where T10, T50 and T90 represent temperatures in xc2x0 F. at which 10%, 50% and 90%, respectively, of fuel has vaporized during ASTM D86 distillation test. It is seen by inspection of the above equation that high volatility fuels have lower DI values and vice versa. In general, fuels with higher DI values, i.e., lower volatility properties, are harder to start and may cause rougher engine operation during warm-up.
Current engine computer control strategy is to calibrate engines for low-volatility fuel (worst case scenario) to avoid potential engine stall and/or driveability problems with lower volatility fuels in the market. This requires significant fuel enrichment during the initial cold-start transient phase of engine control and operation. In such a fuel-blind calibration procedure, the emission levels for high-volatility fuels (used in most emission certification tests) are much higher than would otherwise be possible. On the other hand, had the engine been calibrated for the certification fuel, there could have been driveability problems in real-world vehicle operation and hence potential customer complaints. Thus, an adaptive engine control strategy based on real-world estimation of DI would help both emissions and driveability.
In the absence of a reliable and cost-effective sensor for direct measurement of fuel volatility, optimal fuel control during initial cold-start transients is a difficult task. Without knowledge of how much vaporized fuel is being pumped into the combustion chamber of the engine during cold start, control of the proportions of air and fuel is difficult, if not impossible. With the lack of knowledge of fuel DI, all current engine control strategies have resorted to a conservative high DI calibration base. This assures a fuel rich start-up for driver satisfaction but with increased exhaust emissions. As a result, in practice, on some systems additional costly after-treatment processes (such as the Air Injection Reactor system) are added to the emissions control system to reduce non-methane hydrocarbons (NMHC) and carbon monoxide (CO) to acceptable levels given by the relevant mandated emission standards.
It is clear that there is a significant need for a soft sensor or computer-based process for the real time detection of fuel volatility, especially during cold engine start, to insure simultaneous customer driveability satisfaction and compliance with emissions standards.
This invention provides a generic process for detecting the fuel volatility, or equivalently the driveability index DI, of the gasoline actually being pumped to and injected into the intake port or the combustion cylinders of a vehicle engine during start-up. The invention also provides a compensation process for adjusting or compensating for the low volatility fuel-based calibration with a reliable estimate of the actual volatility of the fuel in establishing the rate of fuel injection to the engine. These detection and compensation processes are preferably carried out within about one second of engine start-up using crankshaft position sensor information (raw rpm data) in the suitably programmed engine control module or powertrain control module of the vehicle.
When a vehicle operator turns the ignition key to start a current gasoline-powered, internal combustion engine (preferably without touching the throttle pedal) under the management of an engine or powertrain control module, the starter motor cranks the engine a few revolutions to enable starting. The powertrain control module (PCM) is activated. It has been pre-calibrated to control the ignition system and air and gasoline flow to start the cold engine. As part of its operation, the PCM receives and processes raw signals from the crankshaft position sensor to time ignition events and ascertain engine start and idle. During the first few events after cranking, fuel is injected into the air inlet ports of all cylinders (asynchronous fuel injection) until the PCM has processed sufficient sensor data to learn crankshaft position. Synchronous fuel injection then begins. The mass of fuel injected per port has been predetermined by the calibration of the engine control designer, but the amount of fuel vapor actually available for combustion in any given engine depends mostly on the actual volatility of the fuel at the ambient temperature.
The proportion of vapor generated by the injected fuel is relatively low in low volatility (high DI) fuels and relatively high in high volatility (low DI) fuels. The PCM has been calibrated to inject a relatively large mass of fuel because of the designer""s assumption that the vehicle may contain a low volatility fuel. The fuel-rich engine quickly starts idling. As soon as the PCM detects the engine run condition, the rate of fuel injection is sharply cut back over a specified number of engine ignition or fuelling events. The engine is now operating in a more fuel-lean condition and engine speed in revolutions per minute (rpm) decreases to a pre-specified level.
As stated, a part of PCM operation involves noting and counting engine events and the time between them. An engine event refers to the timing of ignition or fuel injection into a cylinder. A fuel injection may be into a fuel port for a cylinder or directly into the cylinder. An event period is the time in milli-seconds between two successive injections or ignitions in a multi-cylinder engine.
This invention utilizes the fact that as the PCM alters engine operation from a fuel-rich to a fuel-lean operating regime, there is a drop in the actual amount of fuel vapor available for combustion in the cylinders of the engine. As stated, during the initial transient cold start, the actual fuel vapor in the cylinders of an IC engine depends both on the amount of fuel injection and also on the fuel volatility. Furthermore, it is found that an abrupt cutback in the level of fueling will result in different vapor dynamics for fuels with different volatility.
It is observed that the vapor drop, due to fuel cutback after cranking, is higher for fuels with higher volatility. As the engine torque (or power) level is proportional to the actual level of fuel vapor burned in the cylinders, it had been expected and found by this inventor that there is a larger torque drop (here called xe2x80x9ctorque holexe2x80x9d) for more volatile fuels immediately after an abrupt fuel cutback. Associated with the torque hole is also the corresponding speed drop (with its magnitude called xe2x80x9cspeed droopxe2x80x9d in this specification), the actual value of which is easily measured by timing signals from the crankshaft position sensor. Therefore, in accordance with this invention, the speed droop following an abrupt fuel cutback during the initial cold-start transients is a useful and reliable measure of fuel volatility.
In practice, in all engine cold-start strategies, larger fuel enrichment is initially used to get the engine started. This is then followed by some schedule for fuel cutback. This fuel schedule (internal to the PCM) is exploited in the practice of this invention to detect the fuel volatility without the use of any additional sensors or external probing signals. The detection of fuel volatility from speed droop data in the first second or so after the ignition key is turned on is then used to compensate for actual fuel volatility. Subsequent injection of fuel during the engine and exhaust warm-up period (until the exhaust oxygen sensor is warmed up and closed loop fuel control by the PCM is enabled) is based on the estimated fuel volatility rather than the pre-calibrated fuel-rich value.
With respect to the fuel volatility compensation step, it is preferred to experimentally determine the air/fuel ratio enrichment thresholds for representative fuels over a range of potential cold start temperatures. A calibration table is established for a specific family of engines. This practice is a reliable way of operating the engine control system near its peak potential for reduced emissions without driveability risks. For every engine family and fuel DI (e.g., DI values ranging from 1100xc2x0 F.-1300xc2x0 F. in steps of 25xc2x0 F.) and at any given temperature (xe2x88x9250xc2x0 F. to 120xc2x0 F. with emphasis at 50xc2x0 F.), the optimum A/F enrichment is determined that avoids engine stall and provides good driveability. The data is stored in the form of look up tables in the PCM memory. This is illustrated in the C-curve in FIG. 8.
Speed droops during cold starts for representative gasoline fuels are correlated with their fuel DI values. These DI values and associated speed droops, for a range of applicable temperatures, are also stored in lookup tables accessible by the PCM. For example, see D-curve in FIG. 9. Then during an actual cold start, once the PCM has detected the speed droop following engine start and fuel cutback, the PCM refers to the lookup table for the determination of the proper fuel volatility. Upon estimation of fuel volatility from D-curve table lookups, the PCM uses the C-curve table lookup to determine the optimum fuel/air enrichment. Subsequently, the required degree of fuel enrichment is used by the PCM and the fuel injection is appropriately adjusted.
Other objects and advantages of this invention will become apparent from a detailed description of a preferred embodiment that follows. Reference will be made to figures described in the next section of this specification.